1. The major circulating forms of ANPs in the rat are AlphaR-ANP (5-28) (atriopeptin III) and AlphaR-ANP (3-28) [ANF (8-33)]. 2. Concentrations of ANPs in plasma of conscious rats ranged from 80-120 pg per milliliter. 3. Mechanical and hyperosmotic stimuli enhance the release of ANPs in the conscious rat. 4. Halothane anesthesia elevates concentrations of ANPs in plasma of anesthetized rats. 5. Neuronal influences appear to modulate the volume load-induced release of ANPs from rat atria.